This invention relates to refining of molten metal, and more particularly, to a method and apparatus for removing dissolved gases and other soluble and insoluble impurities from molten aluminum and its alloys.
Molten aluminum, prior to casting, contains many impurities which, if not removed, cause high scrap loss in casting, or otherwise result in poor quality metal products. Typical undesirable impurities requiring removal include dissolved hydrogen, alkali or alkaline earth elements and undissolved non-metallic inclusions.
The injection of inert or reactive gas mixtures into molten aluminum is a commonly used technique for the removal of the above impurities. The rate at which these impurities are removed depends to a great extent on how the fluxing gas is injected into the molten metal. Optimum performance in this type of metal treatment process is achieved when fine gas bubbles are generated creating a large interfacial contact area for the metal treatment reactions to occur, and when these gas bubbles are distributed in a uniform fashion throughout the entire cross-sectional area available for metal flow.
Processes are known in which a rotating impeller is used to inject gas into a body of molten metal without the use of a filter bed. The function of the impellers used in these processes is to generate small gas bubbles, and to distribute them uniformly throughout the entire volume of metal to be treated, or to set up a metal flow pattern such that all of the metal to be treated passes through some portion of the rotating impeller. Processes of that general type are described in U.S. Pat. Nos. 4,634,105; 4,426,068; 4,357,004; 3,870,511; 3,849,119; 3,839,019; 3,767,383 and 3,743,262. These general processes are optimised for the removal of dissolved impurities. They also have some beneficial effect on metal cleanliness by removal of undissolved particulate impurities, or inclusions primarily by flotation. However, reliability of such processes for inclusion removal is variable, due to turbulence on the surface of the treated metal associated with the rotating impeller. Such turbulence tends to re-entrain the inclusions as well as floating dross.
It is known to utilize gas-liquid countercurrent flow within a solid packed bed system to remove non-metallic impurities and hydrogen from molten aluminum. In these systems, the removal of non-metallic inclusions from molten aluminum relies on a countercurrently flowing gas mixture which serves to de-wet the inclusions from the molten metal and improves the filtration efficiency by accumulating the inclusions in the dross layer at the liquid surface. Gas injection typically takes place through a static injection device. Such systems are described in U.S. Pat. Nos. 4,383,888; 3,737,304; 3,373,303 and 3,707,305.
The above mentioned countercurrent gas flow systems have two main disadvantages. Firstly, they are not very efficient for the production of fine, evenly distributed gas bubbles in liquid metals. This is particularly the case in liquid aluminum due to its high surface tension. In addition, the poor wettability of most common refractories by aluminum increases the difficulty of producing a finely dispersed gas-liquid system. When large gas bubbles form, they tend to coalesce as they percolate through the bed, causing high local turbulence, uneven gas-liquid flow distribution, and possibly agitation of the bed itself.
Operational experience with the process disclosed in U.S. Pat. No. 3,737,305 shows that inclusion removal does not require large volumes of treatment gas. Typical treatment gas flow rates used in the process are in the range of 0.20 to 0.30 liters per kg of aluminum treated. The principal concern for inclusion removal is that the treatment gas is equally distributed across the entire fixed bed.
Efficient hydrogen removal, however, typically requires treatment gas flow rates in the range of 0.60 to 0.80 liters/kg. This is a major point of difficulty for processes which utilize static gas injectors beneath a fixed bed. Thus, at the higher gas flow rates required to effect dissolved hydrogen removal, without the turbulent shearing forces provided by a rotary gas injector, the treatment gas bubbles are large and not evenly distributed. Agitation and/or displacement of the fixed bed occurs which reduces significantly the inclusion removal efficiency, and increases dross formation and metal splashing, both of which are undesirable. The maximum practical treatment gas flow rate is limited to relatively low values. Operating conditions listed in U.S. Pat. No. 3,737,305 are a metal flow rate of 800 lb/hr (363 kg/hr) and a metal flow density in the fixed bed of 12 lb/hr/in.sup.2 (equivalent to a bed area of 666.7 in.sup.2 or 4300 cm.sup.2). The gas flow rates are 40 SCFM (18.9 l/min) argon and l SCFM (0.47 l/min) chlorine. This is equivalent to 0.32 liters of treatment gas per kg of metal treated and is equivalent to 0.0045 liters/cm.sup.2 bed/min. It is known that treatment gas flow rates in excess of this value tend to displace the fixed bed for the previously stated reasons.
There is a need for a process which can inject a sufficient volume of treatment gas into a body of molten aluminum below a solid packed bed to remove dissolved hydrogen without unacceptable bed agitation.
Secondly, it is difficult to maintain the gas injectors. Broken or plugged gas injectors can usually only be removed by shutting down the filtration process, and disassembling the filter bed. This is a difficult and expensive procedure, and as a result, the replacement of malfunctioning gas injection equipment is not always carried out with the necessary frequency.
It is an object of the present invention to provide an effective filtration and degassing system in a single unit, which will be more efficient than the prior systems.